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Hour 5new

2.
OBJECTIVES <ul><li>At the end of the lesson, students should be able to : </li></ul><ul><li>Explain dominant and recessive lethal allele. </li></ul><ul><li>Calculate genotypic and phenotypic ratios for recessive lethal alleles (2:1) </li></ul><ul><li>Explain linked genes. </li></ul><ul><li>Show the effect of linked genes on the dihybrid test cross (3:1) in the absence of crossing over. </li></ul><ul><li>Show the effect of linked genes with crossing over on the dihybrid test cross ratio. </li></ul><ul><li>Explain sex-linked genes. </li></ul>

10.
Linked Genes <ul><li>Genes that are on the same </li></ul><ul><li>chromosome are linked. </li></ul><ul><li>Such genes do not obey Mendel’s laws because they do not undergo </li></ul><ul><li>independent assortment. </li></ul><ul><li>They are inherited together unless </li></ul><ul><li>separated by crossing over during </li></ul><ul><li>prophase 1 of meiosis. </li></ul>

12.
SEX DETERMINATION IN HUMANS <ul><li>Human sex is determined by a pair of sex chromosomes called X and Y . </li></ul><ul><li>Because these chromosomes do not look alike, they are called heterosomes . </li></ul><ul><li>All other chromosomes are called autosomes . </li></ul><ul><li>Every human cell contains 23 pairs of chromosomes. </li></ul>

13.
<ul><li>Females have two large X chromosomes ( XX ), males have one X chromosome and one Y chromosome ( XY ). </li></ul><ul><li>During meiosis, the sex chromosomes pair up and segregate into the daughter cells. </li></ul><ul><li>Males are heterogametic sex because they produce different sperm : approximately 50% contain an X chromosome and 50% have a Y chromosome. </li></ul><ul><li>Females produce homogametic sex because all of their egg contain an X chromosome. </li></ul>

16.
Sex Linked Genes <ul><li>Human have 22 pairs of autosomes and a pair of sex chromosome. </li></ul><ul><li>Genes carried on the sex chromosomes are said to be sex-linked . </li></ul><ul><li>Human females have two X chromosomes , meaning they have two sex-linked alleles. </li></ul>

17.
<ul><li>In males, the Y chromosome is smaller and cannot mirror all the genes found on the X chromosome, so males have only one sex-linked allele. </li></ul><ul><li>This is why males suffer from the effects of X-linked genetic diseases more often than females. </li></ul><ul><li>There are no known Y-linked traits, probably because the Y chromosome carries so few genes. </li></ul>